U.S. Pat. No. 5,229,724 in FIG. 1A discloses a probe head for NMR measurements in which at least a first kind of nuclei, namely protons (1H) with a first, higher resonance frequency and a second kind of nuclei, for example 15N or 31P (X) with a second, lower resonance frequency are excited within a magnetic field. The probe head comprises a first input/output (I/O) terminal for feeding a signal of the 1H resonance frequency so as to excite 1H nuclei and to receive, resp., a resonance signal emitted by the 1H nuclei. A second I/O terminal is also provided for feeding a signal of the X resonance frequency so as to excite X nuclei and to receive, resp., a resonance signal emitted by the X nuclei. A measuring coil within the probe head cooperates with a sample. It may surround the sample or be applied to a surface thereof. The measuring coil has a first terminal end and a second terminal end. The first terminal end is coupled to the 1H I/O terminal and the second terminal end is coupled to the X I/O terminal. A stop circuit is tuned to signals of the 1H resonance frequency and is arranged between the second terminal end and the X I/O terminal. The stop circuit comprises a coaxial line having a length equalling a quarter wave length λH/4 of the 1H resonance frequency.
This prior art, hence, utilizes a λH/4 line on the X side of the measuring coil to act as a 1H stop, with one end of the λH/4 line connected to the X side of the measuring coil and the other end being open. The X side also connects to the X I/O terminal. The λH/4 line is, therefore, arranged transversely thereto.
This prior art probe head, therefore, has a first disadvantage that the measuring coil is operated non-symmetrically. The X end of the measuring coil to which the λH/4 line is connected, is namely “cold” for the 1H frequency because the λH/4 line acts as a short. In contrast, the other end of the measuring coil that is connected to the 1H I/O terminal is “hot” for the 1H frequency. This non-symmetry results in inhomogeneities of the high frequency magnetic field within the measuring coil.
A second disadvantage of this prior art probe head consists in that a capacitor is provided directly at the “cold” end of the measuring coil. This capacitor is, hence, directly exposed to the temperature of the measuring coil which may vary within broad ranges when the sample is brought to varying measuring temperatures by means of an appropriate variable temperature control unit. At high temperatures, however, the breakdown voltage or, speaking in more general terms, the rating, in particular the power rating of capacitors goes down. On the other hand, in the field of NMR it is always desired to make measurements at radio frequency power levels being as high as possible. For example, when measurements are made in the area of 0.5 kW, this power level corresponds to a peak voltage of 5 kV at a measuring frequency of 800 MHz or, via the gyromagnetic ratio of the particular kind of nuclei involved, to a magnetic field amplitude of between 100 and 200 kHz.
A third disadvantage of this prior art probe head consists in that the λH/4 line in its orientation transverse to the X signal line results in a construction with a considerable radial dimension. For so-called “wide bore” applications this may be acceptable because there is enough space available within the bore of the cryostat of a superconducting magnet. However, for other applications with a narrow bore, large radial dimensions of a probe head are prohibitive.
In a scientific article of Martin, R. et al. entitled “Design of a triple resonance magic angle spinning probe for high field solid state nuclear magnetic resonance” in Review of Scientific Instruments, Vol. 74, page 3045 (2003) a probe head is disclosed (see FIG. 1(a)) in which a coaxial line, arranged within the X signal line, is directly connected to the lower frequency X side of the measuring coil. However, the 1H stop in this prior art probe head is also configured as a further coaxial line being likewise directly connected to the X side of the measuring coil and transversely to the X signal line. This further coaxial line likewise transforms its open free end as a 1H short or node, resp., having impedance zero (“cold”) to the X side of the measuring coil. In contrast, on the opposite 1H side of the coil, the impedance for 1H is infinite (“hot”). The measuring coil is, therefore also operated asymmetrically.
A similar arrangement is described in a scientific article of Cross, V. R. et al. in J. Am. Chem. Soc., Vol. 98, page 1031 (1976).
It is, therefore, an object, underlying the invention to improve a probe head of the type specified at the outset such that the afore-mentioned disadvantages are overcome. It is a further object underlying the invention to provide a probe head in which the measuring coil can be operated symmetrically. According to another object, the probe head shall be insensitive to high variations in temperature, in particular when variable temperature control units are utilized. Still one more object consists in that the probe head shall have small radial dimensions.